In recent years, the bodies of automobiles have been required to excel in corrosion resistance. For that reason, there has been an increasing tendency to use surface-treated steel sheets showing high corrosion resistance in place of the cold-rolled steel sheets used heretofore.
As such surface-treated steel sheets, galvanized steel sheets deserve the first mention. In the galvanized steel sheets, it is required to increase the amount of zinc to be deposited so as to improve their corrosion resistance. This offers the problems that workability and weldability deteriorate. Steel sheets plated with a zinc alloy to which one or two or more of elements such as Ni, Fe, Mn, Mo, Co, Al and Cr is or are added, or multilayered plated steel sheets have been studied and developed in order to solve such problems. In comparison with said galvanized steel sheets, these steel sheets may be improved in respect of corrosion resistance without causing deterioration of weldability and workability. However, when steel sheets are applied to the bag-structure portions or bends (hemmed portions) of the inner plates of automotive bodies, their surfaces are required to possess high corrosion resistance. A problem with such zinc alloy- or multilayered-plated steel sheets as mentioned above is that their corrosion resistance is still unsatisfactory. As the steel sheets possessing high corrosion resistance, rustproof coated steel sheets applied thereon with a zinc-enriched film have been investigated and developed, as disclosed in Japanese Patent Publication Nos. 45-24230 and 47-6882, and have typically been known under the name of Zincrometal. Even with such rustproof coated steel sheets, however, the coated films may peel off at locations subjected to press-forming, etc., resulting in deterioration of their corrosion resistance. Thus, they are still unsatisfactory for the highly corrosion-resistant, rustproof coated steel sheets needed to meet the requirements of the materials for automotive bodies, etc.
In view of the foregoing considerations and some limitations imposed on the improvements in the performance of the rustproof coated steel sheets by the zinc-enriched films, the present inventors have separately developed steel sheets including thereon protective films in the form of thin films on the order of at most several micrometers and free from any metal powders such as Zn powders, and have proposed them in Japanese Patent Laid-Open Publication Nos. 58-224174, 60-50179, 60-50180 and 60-50181. Such steel sheets are based on zinc or zinc alloy-plated steel sheets, on which a chromate film and the outermost organic composite silicate film are applied, and are found to possess excellent workability and corrosion resistance.
However, later studies made by the present inventors have revealed that as compared to the zinc-enriched film base steel sheet widely used as the rustproof steel sheets for automobiles (for instance, Japanese Patent Publication No. 45-24230), such treated steel sheets as mentioned above are slightly inferior in corrosion resistance in wet environments.
On the other hand, the steel sheets for automobiles have shown a thinning tendency, since it has been intended to reduce the weight of their bodies. As the steel sheets suitable for this, wide use has been made of the so-called bake-hardened steel sheets (the BH type steel sheets) possessing spreadability at an environmental temperature of 120.degree. C. or lower and toughness at 120.degree. C. or higher. For that reason, the film-forming material suitable for such steel sheets should give a complete film at a low temperature of no higher than 150.degree. C., and is required to possess film durability enough to maintain the corrosion resistance of metals. However, the aforesaid coated steel sheets proposed by the present inventors could not be said to possess satisfactory properties in this regard.
With such problems in mind, the present invention has been accomplished for the purpose of providing a highly corrosionresistant, multi-layer coated steel sheet which posseses workability and weldability, has excellent corrosion resistance of uncoated steel sheet, and shows coating adhesion with respect to multi-coating, corrosion resistance coated steel sheet and low-temperature hardenability.